1. Field of the Invention
The invention relates to the packing of polycrystalline silicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon) is predominantly deposited by means of the Siemens process from halosilanes such as trichlorosilane and then comminuted with minimum contamination into polycrystalline silicon chunks.
For applications in the semiconductor and solar industries, chunk polysilicon with minimum contamination levels is desired. Therefore, the material should also be packaged with low contamination before being transported to the customer.
Typically, the polysilicon chunks are packed in single or multiple plastic bags. Usually, they are packed in double bags.
The bags are subsequently introduced into an outer package, for example a large cardboard box, and transported to the customer.
Chunk polysilicon is a sharp-edged bulk material which is sometimes not free-flowing. Therefore, in the packing operation, it has to be ensured that the material does not puncture the customary plastic bags in the course of filling, or even completely destroy them in the worst case.
In order to avoid this, various measures are proposed in the prior art.
US 20100154357 A1 proposes evacuating air out of the bag during the closure operation so as to result in a vacuum of 10 to 700 mbar.
US 20120198793 A1 discloses evacuating air out of the bag before the welding operation so as to result in a flat bag with a low air level.
However, it has been found that these measures are incapable of preventing punctures.
US 20100154357 A1 provides for an energy absorber within the plastic bag during the packing operation, which is supposed to prevent punctures.
Puncturing of the bag can occur, however, not just during the packing operation but also in the course of transport to the customer. Chunk polysilicon is sharp-edged, such that, in the event of unfavorable orientation of the chunks in the bag, relative movement of the chunks to the bag film and pressure of the chunks on the bag film result, respectively, in the chunks cutting through and penetrating the bag film. Chunks protruding from the bag packaging can be unacceptably contaminated directly by surrounding materials, and chunks inside by incoming ambient air.
In addition, in the course of transport of packed silicon chunks, there is unwanted post-comminution as a result of relative movement and collisions, or as a result of edge fracturing and abrasion. This is undesirable especially because the fines formed in the process demonstrably lead to poorer process performance with the customer. The result of this is that the customer has to screen off the fines fraction again prior to further processing, which is disadvantageous.
This problem applies equally to crushed and classified, cleaned and uncleaned silicon, irrespective of the size of package (typically bags containing 5 or 10 kg of polysilicon).
It has been found that the risk of damage to bags increases proportionally with the chunk mass.
One option which is conceivable in principle, that of reducing the puncture rate by reinforcing the bag film, has been found to be of low practicability, especially since such a less flexible film would be more difficult to handle and more expensive.
The main reason for these punctures and also post-comminutions lies in the excessive “freedom of movement” of the bags during transport. During transport (truck, air, sea and train, loading, etc.), there are a number of stresses on the packing unit.
Studies have shown that the most harmful influence here is to be found among the constant vibrations, as caused, for example, to a predominant degree by truck transport.
This problem gave rise to the objective of the invention.